Patent EP-B1-985 334 describes a starter comprising a stator provided with a plurality of magnetic poles arranged on an internal circumferential surface of a yoke. Each magnetic pole is formed by a permanent magnet made of magnetised ferrite such that the North and South poles appear radially. Auxiliary magnets may be provided between said magnets. These auxiliary magnets are made of ferrite and magnetised so that the North and South poles appear circumferentially. A space is provided between each auxiliary magnet and the yoke. An arrangement of this type reduces magnetic leakage.
It has been found that a strong magnetic armature reaction in a direct current electric machine with brushes, especially in a starter having a magnetomotive force (firm) of great intensity at very low voltage and at an output of over 1 kW, may lead to a reduction in the performance of the machine. In certain cases, an angular displacement of the brushes and/or a compensating winding or an auxiliary switching winding are utilised to reduce the effects accompanying the magnetic armature reaction. The angular displacement of the brushes is optimal only for a predetermined electric current. What is more, said winding is usually bulky.
Furthermore, the article entitled ‘The application of Halbach cylinders to brushless AC servo motors’, K. Atallah and D. Howe, IEEE Transactions On Magnetics, Vol. 34, No 4, July 1998, discloses a machine of the ‘Brushless’ type in which the induction variation in relation to the electrical angle in the air gap is sinusoidal. The article ‘New concept of permanent magnet excitation for electrical machines. Analytical and numerical computation’, M. Marinescu and N. Marinescu, IEEE Transactions On Magnetics, Vol. 28, No. 2, March 1992, deals with magnetisation in a machine of the ‘Slotless’ type, the magnetic excitation being either on the rotor or on the stator of the machine.
One of the objectives of the invention is to reduce the effects of the magnetic armature reaction.